According to ANSI C57 test code for testing distribution and power polyphase transformers having a ratio of transformation of 30 to 1 or less, the polarity and phase-relation of a three-phase transformer can be determined by comparing voltages present between the primary and secondary windings. A phase-to-phase voltage of one of the high-voltage windings also is required to be tested. Either the primary or secondary winding may be the high-voltage winding of the three-phase transformer. Also, the transformer may be equipped with terminals mounted on the exterior of the housing of the transformer, or it may be equipped with insulated leads extending from the housing of the transformer, wherein such leads are adapted to have terminal connections made to these leads. Therefore, as used herein, the words "terminal" and "terminals" are intended to be interpreted sufficiently broadly to include "lead" and "leads", respectively.
Three-phase transformers are tested by their manufacturers and also by electric utilities. The tests which are made include polarity and phase-relation, core loss, load loss and transformer ratio. Conventional practice is to make direct connections of test leads of voltmetering circuits to the actual terminals (or leads) of the transformer itself. A voltmetering circuit for measuring relatively high voltages usually includes a potential transformer (PT) plus a voltmeter, as shown in FIG. 6A. A voltmetering circuit for measuring relatively low voltages, as shown in FIG. 6B, sometimes does not require the potential transformer. Another voltmetering circuit for measuring relative high voltages is shown in FIG. 6B. It includes a voltage-divider circuit comprising at least two electrical components, for example two capacitors connected in series between a pair of test leads with a voltmeter connected across one of the components where the voltage is considerably reduced compared with the total voltage across the whole voltage-divider circuit. Thus, as used herein, the term "voltmetering circuit" is intended to be interpreted sufficiently broadly to include a voltage measuring circuit having two test leads, regardless of whether or not a potential transformer or a voltage-divider is included in such circuit.
Also, it is intended that the term "voltage-reducing means" or "voltage-reducing circuit" be interpreted sufficiently broadly to include a potential transformer or a voltage-divider arrangement. Such a voltage-divider may include at least two resistances in series between the test leads, two capacitances in series, or other suitable electrical components or combinations of electrical components in series between the two test leads.
Set forth below is a portion of the ANSI C57 test code showing Transformer Lead Markings and Voltage-Phasor Diagrams for Three-Phase Transformer Connections for performing polarity and phase-relation tests. In these diagrams "H" indicates the terminals (or leads) on the high-voltage side of the three-phase transformer, and "X" indicates the terminals (or leads) on the low-voltage side of the transformer. The respective three high-voltage terminals (or leads) are H1, H2 and H3, and the respective three low-voltage terminals (or leads) are X1, X2 and X3. ##SPC1##
The present invention relates to the testing of distribution, power and regulating transformers (three-phase transformers). As used herein, the term "Electrical Power Transformer" is intended to be interpreted sufficiently broadly to include all of these three-phase transformers.
In the above diagram each dash (-) means a voltage difference. Thus, for example, "H2-X2" means the voltage difference between high-voltage terminal (or lead) H2 and low-voltage terminal (or lead) X2. In the above diagram for making such "CHECK MEASUREMENTS", it is to be noted that high-voltage terminal (or lead) H1 is shown connected to low-voltage terminal (or lead) X1. This testing connection between H1 and X1 is called a "referencing connection" in the present specification. It is to be noted that when this referencing connection is in place between H1 and X1, there are some transformers which cannot be fully excited during testing, because full excitation might sometimes possibly cause failure of dielectric material inside of such a reference-connected transformer.
In accord with the ANSI C57 test code, the polarity and phase-relation tests are conventionally made by connecting each voltmeter and its potential transformer (PT) (or other voltage-reducing means) as shown by FIGS. 1 through 4 of the present specification. These FIGS. 1 through 4 each shows five such voltmetering circuit connections as conventionally arranged for the four respective types of three-phase Electrical Power Transformers, namely: delta-delta, wye-wye, delta-wye and wye-delta. It is to be noted that there is no neutral ("ground") connection in any of these conventional voltmeter and voltage-reducing circuit arrangements as shown in FIGS. 1 through 4 for polarity and phase-relation testing.
As used herein, the terms "neutral", "neutral connection", "ground" and "ground connection" are all intended to mean a connection having no significant electrical potential difference from the Earth. In other words, there is no significant potential difference between a neutral or a neutral connection on one hand and a ground or a ground connection on the other hand.
In performing the core loss, load loss and transformer ratio tests, it is conventional to use six delta-connected voltmetering circuit connections, as shown in FIG. 5A. It is to be noted that there is no neutral connection in these conventional delta-connected voltmetering circuits. Alternatively, it is conventional to use six wye-connected potential transformers whose primary windings are respectively connected as shown in FIG. 5B to the six respective transformer terminals (or leads) H1, H2, H3, X1, X2 and X3. The six voltmeters in such a prior conventional testing arrangement are not connected to neutral. For purposes of illustration, FIGS. 5A and 5B as an example, show a delta-delta transformer being tested. It is to be understood that core loss, load loss and transformer ratio tests for wye-wye, delta-wye and wye-delta transformers also are conventionally performed by using six voltmetering circuits delta-connected to the six respective transformer terminals (or leads) H1, H2, H3 and X1, X2, X3 as is shown in FIG. 5A or alternatively by using six potential transformers wye-connected in the same manner as shown in FIG. 5B to the six respective three-phase transformer terminals (or leads) H1, H2, H3, X1, X2 and X3 with six voltmeters connected to the respective secondary windings of the potential transformers as is shown in FIG. 5B; therefore, it would be needlessly repetitive to show such conventional voltage-testing circuit connections for the latter three types of three-phase transformers, since such connections would all be the same as is shown either in FIG. 5A or 5B.
In accordance with conventional practice for performing polarity and phase-relation, core loss, load loss, and transformer ratio tests, the test operator follows either of two procedures:
1. In the first conventional testing procedure only a few voltmetering circuits are used by the test operator. In an extreme situation only one voltmetering circuit would be used. The operator manually connects the voltmetering circuits across a few pairs of terminals whose voltages are required to be determined. Then, these voltmetering circuits are connected across another few pairs of terminals. Then, these voltmetering circuits are connected across additional few pairs of terminals, and so forth, until all of the required voltages have been recorded.
2. In the second conventional testing procedure, the operator pre-connects ten or eleven voltage measuring circuits across the desired pairs of terminals of the three-phase transformer depending upon potential transformer configurations or other voltage-reducing circuit configurations. In other words, five voltmetering circuits are connected as shown in any one of FIGS. 1 through 4, depending upon which of the four types of three-phase transformers is being tested, and in addition, five or six more voltage measuring circuits are connected as shown in one of FIGS. 5A or 5B. Thus, in this second procedure ten or eleven voltage measurements are made simultaneously.
As further background, it is noted that in performing the core loss test described herein, the usual procedure is to excite the transformer under test by feeding electrical power into the low-voltage side of the transformer, regardless of whether the low-voltage side is the primary or secondary side. Among reasons for feeding energy into the low-voltage side during core loss testing is the greater ease, convenience and safety of handling electric power at lower voltages rather than at higher voltages.
In performing the "load loss" test, the operator usually connects "shorting bars" across the three low-voltage terminals X1, X2 and X3, so that the terminals (or leads) of the respective three low-voltage windings are all short-circuited together. For performing load loss tests, it is desirable to feed current into the high-voltage side because lesser current is required to be fed into the transformer, and so the low-voltage side is usually the "shorted" side.
Problems associated with the first test procedure using only a few voltmetering circuits arise from the fact that large amounts of labor are required. The test operator opens the circuit breakers at the test panel for deenergizing the transformer and then walks out to the transformer and connects the voltmetering circuits to a few pairs of terminals and then walks back to the test panel and closes the circuit breakers a first time for energizing the transformer for making a first few voltmetering measurements. Then, the circuit breakers are opened, and the operator walks out to the transformer to connect the voltmetering circuits to another few pairs of terminals, and then the operator walks back to the test panel and closes the circuit breakers a second time for energizing the transformer for making a second few voltmetering measurements. Consequently, the operator is being called upon to make numerous repetitive walks out to the transformer and back again. In the extreme case using only one voltmetering circuit up to eleven such walks may occur. Each time the operator must be sure to open the circuit breakers before approaching the transformer. Also, each time the operator must be sure that the voltmetering circuits are being connected to the appropriate pairs of terminals (or leads).
The operator can avoid the trouble and hassle of repetitive walks back and forth between the test panel and the transformer by leaving the transformer in its energized condition, i.e., by leaving the transformer electrically "hot". Then, the operator uses self-protective insulating equipment including two long insulating rods (often made of fiberglass or epoxy-impregnated wood) for contacting the two test leads of the voltmetering circuit to the respective desired pairs of terminals (or leads) of the "hot" transformer in a desired sequence for making the various voltage measurements. Making any such sequence of voltage measurements on a "hot" three-phase transformer is dangerous and hazardous if the operator is not alert.
Problems associated with the second test procedure using up to eleven voltmetering circuits arise from the fact that a large amount of expensive equipment is needed. Moreover, eleven voltmetering circuits have a total of twenty-two test leads. Each of these twenty-two test leads must be connected appropriately to the respective transformer terminals (or leads). Handling and organizing such a relatively large number of test leads all of which must be pre-connected to the required respective six terminals (or leads) of a three-phase transformer invite confusion and difficulties in measurements and possible inaccurate or incorrect test results.